1. Field of the Invention
The present invention relates to a motor drive device for controlling the driving of a motor, and to an electric apparatus using the same.
2. Description of Related Art
In a motor drive device for controlling the driving of a motor, in order to set the rotation speed of the motor to a desired value, a drive voltage applied to one end of a motor coil (and thus a drive current flowing through the motor coil) needs to be controlled to be a desired value.
Accordingly, conventional motor drive devices have commonly adopted a configuration in which, as shown in FIG. 13, bipolar transistors Tr1 to Tr4 are used as switch elements in an H-bridge circuit, and that is provided with transistors Tra and Trb and constant current sources Ia and Ib for controlling the base currents of upper-side transistors Tr1 and Tr2 according to a control voltage Vref externally applied as a drive voltage setting signal, thereby a drive voltage applied to one end of a motor coil L (and thus a drive current flowing through the motor coil L) being controlled to be a desired value (a so-called dropper-type regulator configuration). A control circuit CNT in FIG. 13 controls the on/off states of the transistors Tr1 to Tr4 (i.e., the operation mode of the motor) according to the logics of externally fed operation mode control signals FIN and RIN (binary signals).
In the motor drive device configured as described above, while the upper-side transistor Tr1 (Tr2) is kept on, a drive voltage (a voltage having approximately the same value as the control voltage Vref) obtained by raising the control voltage Vref only by the value of 1Vf of the transistor Tra (Trb) and then dropping the control voltage Vref only by the value of 1Vf of the transistor Tr1 (Tr2) is applied to the one end of the motor coil L.
As other conventional arts related to the present invention, there have been disclosed and proposed: a motor drive control system in which field effect transistors are used as switch elements in an H-bridge circuit and gate voltages of the transistors are controlled by pulse width modulation control (hereinafter referred to as PWM (pulse width modulation) control) (see JP-B2-3665565 (hereinafter referred to as Patent Publication 1)); and an art in which, with respect to a triangular wave generating circuit that outputs the charge/discharge voltage of a capacitor as a triangular wave, the frequency of the triangular wave is maintained at a predetermined frequency by controlling such that the amplitude of the triangular wave is reduced with reduction of the power supply voltage, and further, the value of a current for charging and discharging the capacitor is reduced according to this amplitude (see JP-A-2002-223563 filed by the applicant of the present application (hereinafter referred to as Patent Publication 2)).
Conventionally, as means for maintaining the rotation speed of a DC brush motor at a desired value, bridge servos (see FIG. 14A) and proportional current servos (see FIG. 14B) have been widely used.
As another conventional art related to the present invention, the applicant of the present invention has disclosed and proposed in JP-A-H06-245573 (hereinafter referred to as Patent Publication 3) an electronic-governor-type DC-motor control circuit including a resistor bridge for controlling a DC motor that drives an audio signal recording medium. This DC-motor control circuit includes a PWM-type control circuit that has a predetermined constant frequency which is beyond the audible range and whose pulse width is modulated according to the rotation state of the DC motor detected by a resistor-bridge circuit including the resistor bridge, and this control circuit switches power supplied to the DC motor so as to control the rotation speed of the DC motor.
It is true that, with a motor drive device having the conventional configuration shown in FIG. 13, it is possible to control the drive voltage applied to one end of a motor coil L (and thus the current voltage flowing through the motor coil L) to be a desired value according to a control voltage Vref, regardless of the power supply voltage Vcc, by appropriately generating the control voltage Vref by use of a resistive divider circuit or the like provided outside the device.
However, in the conventional motor drive device described above, for the purpose of maintaining proper operation of the transistor Tra (Trb), the control voltage Vref can be set, at the highest, to a voltage value given by subtracting the value of 1Vf of the transistor Tra (Trb) from the power supply voltage Vcc (in practice, an even lower voltage value determined with even the voltage drop occurring at a constant current source Ia (Ib) taken into consideration). As a result, the drive voltage that can be applied to one end of the motor coil L is lower than the power supply voltage Vcc at least by the value of 1Vf of the transistor Tra (Trb), and this prevents an effective use of the power supply voltage range.
In addition, in the conventional motor drive device described above, since the bipolar transistors Tr1 to Tr4 are used as the switch elements of the H-bridge circuit, the response of the switch elements to on/off control is not necessarily fast.
Patent Publication 1 discloses nothing more than an art in which the operation mode of a motor is selectively set by PWM driving the switch elements of an H-bridge circuit according to a drive current flowing through a motor coil, and does not disclose anything with respect to an art in which the drive voltage applied to one end of a motor coil is set to a desired value according to an externally applied control voltage.
The conventional art of Patent Publication 2 relates to a switching regulator that is PWM-driven by use of a triangular wave generated by a triangular wave generating circuit, and its main object is limited to providing a switching regulator that is capable of securely performing PWM driving according to variation in the output voltage by preventing PWM driving capability from deteriorating even when the power supply voltage is lowered and ensuring a predetermined duty ratio even when the power supply voltage is lowered. Therefore, the conventional art of Patent Publication 2 hardly helps to solve the above described problems.
As to conventional arts for maintaining the rotation speed of a DC brush motor at a desired value, the bridge servo and the proportional current servo shown in FIGS. 14a and 14B, respectively, monitor the terminal voltage of a motor M (and thus the back electromotive voltage EBEMF generated in proportion to the rotation speed of the motor M), and control a power supply voltage Vcc or a motor current IM such that the terminal voltage of the motor M is equal to a predetermined control voltage Vref. The bridge servo and the proportional current servo make it possible to maintain the rotation speed of the motor M at a desired value with a simple configuration, but on the other hand, they generate an inconveniently large amount of heat or consume an inconveniently large amount of power when the motor M is driven with a large current.
With the conventional art of Patent Publication 3, it is possible to substantially reduce power consumption by using a switching regulator to control the power supply voltage Vcc. However, with the conventional art of Patent Publication 3, an expensive coil is required as one of its circuit elements, and moreover, the rotation speed cannot necessarily be set easily.
The bridge servo and the proportional current servo shown in FIGS. 14A and 14B, respectively, are designed on condition that one end of the motor M is connected to a power supply line, and a couple of the above mentioned servos need to be provided in order to freely switch the rotation of the motor M between the forward and reverse rotations. This causes increase in circuit size and in cost.